1. Field of the Invention
The present invention relates to a composition of a lubricating oil for internal combustion engine such as an engine oil for gasoline automobiles. More particularly, the present invention relates to a lubricating oil for internal combustion engine which can retain the fuel-saving effect for a longer period of time.
2. Related Art
From a viewpoint of suppression of CO.sub.2 content in the atmosphere which is considered one of the causes for making the earth warmer as well as resource-saving, the engine oils for the gasoline automobiles (hereinafter referred to briefly as "engine oil") have been strongly required to possess low fuel economy in addition to performances such as wear resistance, oxidation stability, detergency and dispersancy.
Ordinarily, the engine oil is composed of a mineral oil purified from petroleum, or a synthetic lubricating oil such as .alpha.-olefin oligomer and ester, added with additives such as a detergent, a dipersant, an antioxidant, an anti-wear agent, and a viscosity index improver. In order to increase the fuel efficiency, it is effective to lower the viscosity of the engine oil. However, mere lowering of the viscosity increases a boundary lubricating area, which leads to increase in friction in some cases. For this reason, a friction modifier (FM) has recently come to be added to the engine oil so as to reduce friction in the boundary lubricating area. Among friction modifiers, organometallic additives are more effective than ashless additives such as ester, amine and amide additives. It is known that, among these, organic molybdenum compounds such as molybdenum dithiocarbamate (MoDTC) and oxymolybdenum organo phosphorodithioate sulfide (MoDTP) are highly effective as described in Japanese Patent Publication of Examined Applications (JP45-B) No.3-23595. It is also reported that the use of MoDTC together with zinc dithiophosphate (ZnDTP) affords the higher friction decreasing effect.
However, the use of the engine oil in the course of time is accompanied by deterioration of and consumption of MoDTC. Therefore, there is a problem that, although a fresh engine oil gives a high fuel economy, such a high fuel economy of the engine oil is deteriorated with the lapse of time. In order to solve the above problem, it can be considered that the addition amount of MoDTC in a fresh oil is increased. However, since MoDTC having a relatively short alkyl group has in general lower solubility, the increase in the addition amount reduces the storage stability. On the other hand, since MoDTC having a relatively long alkyl group has higher solubility but lower thermal resistance, it can hardly be used for a longer period of time.
Accordingly, the object of the present invention is to provide a lubricating oil for internal combustion engine which retains friction loss thereof at lower level even after long use without occurrence of a precipitate derived from MoTDC produced by a new process prior to use, suppress the precipitation of MoDTC and the occurrence of sludge at lower level, and produces no deterioration as to quality even when stored for a longer period of time.